FIG. 1 shows example structures of a conventional encoding apparatus 1 and a conventional decoding apparatus 2.
An acoustic time-sequential signal SS sent to the encoding apparatus 1 is input to an amplitude control process section 11.
The amplitude control process section 11 divides the input acoustic time-sequential signal SS into blocks (having a fixed length) each of which serves as an encoding unit, further divides each block into a plurality of sub-blocks, and analyzes the amplitude of the acoustic time-sequential signal SS in each sub-block. According to the result of the amplitude analysis, the amplitude control process section 11 generates amplitude control process information G used in an amplitude control process for making an amplitude almost constant in a block. When the acoustic time-sequential signal SS in a block includes a high-frequency component locally generated, as shown in FIG. 2A, for example, the amplitude control process section 11 recognizes that the amplitude changed from an amplitude H1 to an amplitude H2 at a time t0, and from the amplitude H2 to the amplitude H1 at a time t1, and generates amplitude control process information G indicating the state.
The amplitude control process section 11 outputs the generated amplitude control process information G to a code-string generation section 14, and processes the amplitude of the acoustic time-sequential signal SS in each sub-block according to the amplitude control process information G. Specifically, the amplitude control process section 11 generates a time-sequential signal GS having an almost constant amplitude in each block, and sends it to a spectrum conversion section 12.
In a case shown in FIG. 2, the process process section 11 applies an amplitude control process to the acoustic time-sequential signal SS in a block shown in FIG. 2A so as to amplify the amplitude H1 until the time t0 and the amplitude H1 after the time t1 to the amplitude H2, and outputs the time-sequential signal GS obtained as a result, like that shown in FIG. 2B, and having an almost constant amplitude in the block, to the spectrum conversion section 12.
The spectrum conversion section 12 decomposes the time-sequential signal GS sent from the amplitude control process section 11 into a spectrum F formed of frequency components according to MDCT (modified discrete cosine transform) or others, and outputs the spectrum F to a normalization/quantization section 13.
The normalization/quantization section 13 analyzes the spectrum F sent from the spectrum conversion section 12 to generate normalization information N. The normalization/quantization section 13 outputs the generated normalization information N to the code-string generation section 14, and normalizes the spectrum F according to the normalization information N.
The normalization/quantization section 13 also outputs predetermined quantization information Q to the code-string generation section 14, quantizes the normalized spectrum F according to the quantization information Q, and outputs spectrum information QF obtained as a result to the code-string generation section 14.
The code-string generation section 14 encodes the amplitude-control process information G sent from the amplitude control process section 11 and the normalization information N, the quantization information Q, and the spectrum information QF sent from the normalization/quantization section 13 to generate a code string CD. The generated code string is sent to the decoding apparatus 2.
The code string CD sent to the decoding apparatus 2 is input to a code-string decomposition section 21.
The code-string decomposition section 21 decomposes the input code string CD into the spectrum information QF, the quantization information Q, the normalization information N, and the amplitude control process information G, and outputs the spectrum information QF, the quantization information Q, and the normalization information N to a dequantization/denormalization section 21, and the amplitude control process information G to an inverse amplitude control process section 24.
The dequantization/denormalization section 22 dequantizes the spectrum information QF input from the code-string decomposition section 21 according to the quantization information Q, then, denormalizes it according to the normalization information N, and outputs a spectrum F′ obtained as a result and corresponding to the spectrum F generated in the encoding apparatus 1 (spectrum conversion section 12) to an inverse spectrum conversion section 23.
The inverse spectrum conversion section 23 applies inverse spectrum conversion to the spectrum F′ sent from the dequantization/denormalization section 22, according to IMDCT (inverse MDCT: inverse modified discrete cosine transform) to generate a time-sequential signal, and outputs the time-sequential signal GS′ obtained as a result and corresponding to the time-sequential signal GS generated by the encoding apparatus 1 (amplitude control process section 11), to the inverse amplitude control process section 24.
For example, the time-sequential signal GS′ corresponding to the time-sequential signal GS shown in FIG. 2B is generated, and output to the inverse amplitude control process section 24.
The inverse amplitude control process section 24 applies an amplitude control process based on the amplitude control process information G sent from the code-string decomposition section 21, that is, an amplitude control process for processing an amplitude inversely to the amplitude control process performed in the encoding apparatus 1 (amplitude control process section 11), to the time-sequential signal GS′ sent from the inverse spectrum conversion section 23, and outputs an acoustic time-sequential signal SS′ obtained as a result and corresponding to the acoustic time-sequential signal SS input to the encoding apparatus 1, to the outside.
For example, the acoustic time-sequential signal SS′ corresponding to the acoustic time-sequential signal SS shown in FIG. 2A is generated and output to the outside.
As described above, since a portion having a small amplitude (such as portions having the amplitude H1 in the acoustic time-sequential signal SS shown in FIG. 2A) in the acoustic time-sequential signal SS is amplified, and encoded after the amplitude control process (such as that shown in FIG. 2B) for making an amplitude almost constant in a block is performed, in the encoding apparatus 1, and the amplitude control process for making the amplified portion to the original state is performed and a code string is decoded in the decoding apparatus 2, the occurrence of pre-echo and post-echo, which can be heard when quantization noise is not masked by a small-amplitude signal, is suppressed.
When the acoustic time-sequential signal SS in a block is a low-frequency-component time-sequential signal having a large amplitude H11, like that shown in FIG. 3A, which includes a high-frequency component occurring at a partial time, like that shown in FIG. 3B, for example, the encoding apparatus 1 (amplitude control process section 11) recognizes that the amplitude in the block is constant at the amplitude H11 by amplitude analysis applied to the acoustic time-sequential signal SS.
More specifically, since the amplitude control process for amplifying a portion having a small amplitude is not performed, and a time-sequential signal GS similar to the acoustic time-sequential signal SS (shown in FIGS. 3A and 3C) is generated, spectrum-converted, normalized, quantized, and encoded in the encoding apparatus 1, and the amplitude control process for making the amplitude back to the original state is not performed and a code string is decoded in the decoding apparatus 2, an acoustic time-sequential signal SS′ like that shown in FIG. 3C, including a diffused high-frequency-component quantization noise like that shown in FIG. 3D, is generated, and pre-echo and post-echo are heard at portions enclosed by dotted lines in FIG. 3C, for example, without being masked.
As described above, the conventional encoding apparatus 1 or the conventional decoding apparatus 2 does not appropriately suppress pre-echo or post-echo.